Electronic flash unit

ABSTRACT

A quench tube is connected in parallel with the flash tube. A control signal for initiating the flash or the quenching operation is derived selectively from a measuring and control circuit by the operation of a switch. Thus the unit may be used alternatively to provide automatic flash control or to initiate the flash in response to light from another flash unit.

I United States Fatem [151 3,636,406 Ackermann [4 1 Jan. 18, 1972 [54]ELECTRONIC FLASH UNIT 3,350,603 10/1967 Erickson ..3 15/241 P 3,487,22112/1969 Frank ...315/24l P 1 lnvemofl Karl Ackermflnn, Berlm, Germany3,517,255 6/1970 Hoffer et al ..315/151 [73] Assignee: Robert BoschPhotokino GmbH, Stuttgart- Umenurkheimy Germany Primary Examiner-RoyLake Assistant Examiner-Lawrence J. Dahl [22] Filed: Dec- 15, 1969Att0rneyMichael S. Striker [2]] App]. No.: 884,794 [57] ABSTRACT Aquench tube is connected in parallel with the flash tube. A [30] ForeignApplication Monty Dam control signal for initiating the flash or thequenching opera- Apr. 19, 1969 Germany ..P 19 20 036.3 tion is derivedselectively from a measuring and control circuit by the operation of aswitch. Thus the unit may be used alter- [52] U.S. Cl ..3l5/24l, 320/1natively to provide automatic flash control or to initiate the [51] Int.Cl. ..H05b 41/00 flash in response to light from another flash unit.[58] FieldofSearch ..3l5/151,241,241P; 320/1 10 Claims, 2 DrawingFigures [56] References Cited UNITED STATES PATENTS 3,340,426 9/1967 ljll liott 315/241 X PATENTEnJAmemz 3.636.408

sum 1 BF 2 lm enfan- BACKGROUND OF THE INVENTION This invention relatesto an electronic flash unit. In particular, it relates to electronicflash units with automatic control of the duration of a flash. In thistype of flash unit, the flash tube is connected in parallel to a quenchtube. A control signal applied to the quench tube causes a substantialshort-circuiting of the flash tube, thus terminating the light flash. Inthe conventional units, the control signal causing the termination ofthe light flash, is furnished by a measuring and control circuit. Themeasuring and control circuit comprises a photoelectric element whichfurnishes a signal corresponding to the amount of light utilized in thephotographic process, for example, the amount of light reflected backfrom an object to be photographed. The photoelectric signal, for examplea current, is then integrated, for example by use of a capacitor. Whenthe total amount of light reflected from the object reached apredetermined value, a control signal is furnished by the measuring andcontrol circuit. This control signal is applied to an ignition circuitof the quench tube, thus causing the termination of the light flash.

By use of the above-mentioned equipment, it is possible to operate at afixed lens opening, since the correct illumination of the object isachieved by the above-described termination of the flash, which in turnis initiated when the object to be photographed has been properlyilluminated.

The photoelectric element mentioned above may for example be aphotoresistance whose resistance varies as a function of illuminationfalling thereon. Such a resistance may be utilized in a circuit whereinthe current flow is a function of said resistance. The current may inturn be used to charge a capacitor, the control signal being furnishedwhen the voltage on the capacitor has reached a predetermined value. Ofcourse, the resistance offered by the quench tube after ignition must below compared with the resistance of the flash tube after ignition, orthe short-circuiting of the flash tube by the quench tube will not causesufficient energy to be removed from the flash tube in order toterminate the flash.

Another class of flash units is known, which difi'ers from the basicflash unit without automatic control in that a photoelectric elementfunctions in conjunction with an electronic control circuit in order topermit initiation of a flash operation. The photosensitive part of thephotoelectric element is mounted on the front surface of the housing ofthe flash unit for this purpose. For example, another flash unit (themain flash unit) may be generating a light flash in another location.The flash generated by this main flash unit falls upon the photoelectricelement of the flash unit under discussion, causing theadditionalelectronic control circuit to furnish a control signal for initiatingthe flash, that is, the control signal is applied to the ignitioncircuit of the flash tube. In this manner, it is possible to illuminatethe object to be photographed by two flash units, without therequirement that the two units are connected by an electric cable, andpermitting the two units to be located in different locations.

SUMMARY OF THE INVENTION The basic consideration underlying the presentinvention is, that it should be possible to furnish an electronic flashunit which combines the two above-described functions, namely theautomatic termination of a flash originating from the unit, and theinitiation of the flash in the unit in response to externally fur ished.light, within a single electronic flash unit.

It is therefore an object of the present invention to furnish such acombined flash unit with a minimum of additional components as comparedto a flash unit operating to fulfill only one of the above-describedfunctions.

The present invention thus comprises an electronic flash unit which hasIight-flash-generating means for generating a light flash in response toa control signal applied to said lightflash-generating means. The flashunit further comprises terminating means connected to saidlight-flash-generating means for terminating said light flash inresponse to a control signal applied to said terminating means. Furtherfurnished is a measuring and control circuit having a photoelectricelement mounted to receive light indicative of the light utilized in thephotoelectric process. This measuring and control circuit is adapted tofurnish a control signal when the quantity of light falling on saidphotoelectric element has reached a predetermined value. Finally,switching means are provided for selectively interconnecting saidmeasuring and control circuit means and said light-flash-generatingmeans or said terminating means, whereby said measuring and controlcircuit means functions selectively to terminate the flash generated bythe light-flash-generating means or to initiate a light flash inresponse to externally furnished illumination.

The Iight-flash-generating means may be a flash tube, while theterminating means may be a gaseous discharge device connected inparallel with the flash tube and adapted to short circuit said flashtube when a control signal is applied to the quench tube ignitioncircuit.

An electronic flash unit with the above characteristics differs from aunit with automatic control of the duration of the flash simply by theinclusion of a single pole switch and its interconnecting wiring. Thisof course represents an extremely small amount of equipment relative tothe total equipment required for such a unit. Further, the amount ofspace occu pied by this additional equipment is also negligible, causingthe combined unit to have substantially, or even precisely the same sizeas the unit having only automatic control of the flash duration.

In a preferred embodiment of the present invention, the control voltage,or output of the measuring and control circuit means, is appliedselectively to the ignition circuit of the quench tube or the ignitioncircuit of the flash tube, in dependence on the use to which the flashunit is being put. Furthermore, it may be advantageous that theswitching means also function to select the operating voltage source forthe measuring and control circuit in dependence on the use to which theflash unit is being put.

This possibility of switching the operating voltage in dependence uponthe use to which the flash tube is being put, must be provided wheneverit is essential that the measuring and control circuit means remaininsensitive to flashes emanating from other electronic flash units; thatis, when the flash unit is being used to automatically terminate theflash, and it is desired that no light furnished by other flash unitsinfluence determination of the light flash in the unit which is thesubject of this invention. For this type of operation, it is known thatthe operating voltage for the measuring and control circuit may befurnished only during the duration of the flash, as, for example, by avoltage developed across a resistance which is electrically connectedinto the circuit of the flash tube. The value of the resistance must besuch that the voltage developed across it while the flash tube isignited, is sufficiently large to constitute the operating voltage forthe measuring and control circuit.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTThe preferred embodiment of the present invention is now going to bediscussed in relation to the drawing.

Numeral I, in FIG. 1, designates a storage capacitor associated with theelectronic flash unit, whose terminals 2 and 3 are connected to a directcurrent high-voltage source which is not illustrated in the drawing. Aflash tube, one embodiment of light-flash-generating means, has a firstelectrode 4 connected to what may, for example, be positive tenninal 2,and a second electrode 5, connected to what may, for example, be thenegative terminal 3 of capacitor 1. The flash tube ignition circuit,namely, the ignition circuit for flash tube 6, comprises a capacitor 7which is charged to a voltage which is a fraction of the voltage ofcapacitor 1, and ignition transformer 10 having a primary winding 8 anda secondary winding 9, and a switch 11 which may be operated manually orthrough the contact of the camera in order to initiate a flash. Terminal12 of secondary winding 9 is connected to ignition electrode 13 of theflash tube. A connecting line 14 connects the positive terminal 2 to oneend of a voltage divider comprising a series combination of resistors 17and 18, while a line connects the negative terminal 3 to the otherextreme of the voltage divider. The resistor 18 is connected in parallelto a capacitor 19. Resistors I7 and 18, in conjunction with capacitor19, constitute the first operating voltage-fumishing means forfurnishing an operating voltage to the measuring and control circuitmeans, which will be described below. The common point 20 of resistors17 and 18, is connected to a terminal 21 which, together with twomechanically coupled switching arms 22 and 23 constitute part of theswitching means. The dashed line 24 in FIG. 1 indicates the mechanicalcoupling between the arms 22 and 23. As indicated in FIG. 1, a contact25 may be selectively connected via switching arm 22 to one of threecontacts labeled S, O, or A. In FIG. 1, the switching arm 22 connectsterminal 25 to terminal S which is the position corresponding to theoperating condition in which the flash in tube 6 is initiated by a flashemanating from another flash unit. The actual measuring and controlcircuit means, labeled 16 in FIG. 1, comprise a diode 26 having an anodeconnected to terminal 25 and a cathode connected to a capacitor 27 whoseother terminal is connected to the negative supply terminal. The cathodeof diode 26 is further connected to a photoelectric element 28, whichmay for example be a photoresistance. A capacitor 29 which serves as anintegrating capacitor, is connected to the other terminal of thephotoelectric element 28. The other terminal of capacitor 29 isconnected also to the negative supply terminal. A potentiometer 30 isconnected parallel to capacitor 29. The wiper arm 31 of thepotentiometer is connected to the anode of a diode 32 whose cathode isconnected to the negative supply terminal via a resistance 35. Thecathode of diode 32 is also connected to the control electrode 33 of athyristor 34. The cathode of thyristor 34 is connected to the negativeterminal 3, while the anode is connected to a fixed voltage +U, which isderived from the voltage across the storage capacitor 1. The anode ofthyristor 34 is further connected to an ignition capacitor 35, whoseopposite terminal is connected to a terminal 36 which is also part ofthe switching means. The movable arm 23 serves to connect the terminal36 selectively to a terminal S, O, or A. In FIG. 1, terminal 36 is shownconnected to terminal S, as was mentioned above, which is alsodesignated by the numeral 37. Terminal 37 is connected by line 38 to theprimary winding 8 of the ignition transformer 10.

The circuitry described above is, as mentioned above, designed toinitiate a flash in response to light received from another flash unitand fulfills the following functions: After a short charging time, thevoltage across capacitor 1, that is the voltage appearing betweenterminals 2 and 3, and thus between electrodes 4 and 5 of the flash tube6, is a direct voltage of, for example, 500 volts. Initially, no voltageappears across the relatively low resistor 29, since the flash tube isnot ignited. The measuring and control circuit receives an operatingvoltage, namely the voltage appearing across resistance 18 and smoothedby means of a capacitor 19, which is applied to capacitor 27 via themovable contact arm 22 and diode 26.

This capacitor thus charges to its maximum voltag'e. A quiescent currentflows through the at first unilluminated photoresistance 28, causing asmall charge to appear across capacitor 29. Part of the voltageappearing across capacitor 29 is applied to control electrode 33 ofthyristor 34. The particular portion of the voltage to be applied to thecontrol electrode is determined in advance by the setting of thepotentiometer 30. This setting may, for example, correspond to thesensitivity of the particular film used. The anode-cathode circuit ofthyristor 34 is initially nonconductive, since the voltage appearingbetween the cathode of the thyristor and its control electrode 33, istoo low when the photoresistance is unilluminated to allow the thyristorto become conductive. As long as thyristor 34 is blocked, capacitor 35,which is connected to the voltage U cannot discharge and retains itscharge completely.

If now light originating from another flash unit falls ontophotoresistance 28, this photoresistance has a decreasing resistancevalue, thus permitting a larger current to flow from capacitor 27 to theintegrating capacitor 29. Capacitor 29 therefore charges to a highervoltage which serves as the control voltage for thyristor 34 and causesthe thyristor to become conductive. This in turn permits the ignitioncapacitor 35 to discharge via the anode-cathode circuit of the thyristorand the primary winding 8. Thus a current pulse appears in the primarywinding 8 which causes the flash tube 6 to ignite, since it causes acorresponding current pulse to appear in the secondary winding 9 andthus at the ignition electrode 13.

In another switching position, called the third switching positionherein, the contact 22 is connected to the contact 0. In this position,the relevant parts of the measuring and control circuit, and inparticular the photoresistor 28 and thyristor 34 are disconnected, sincethe electrical connection between terminals 21 and 25 is broken. Theelectronic flash unit may then operate like an electronic flash unitwithout automatic control and without the ability to initiate a flash inresponse to an externally furnished light. That is, a flash can beinitiated by means of the closing of contact 11, which may be affectedmanually or via a contact in the camera. This causes the previouslycharged capacitor 7 to discharge over the momentarily closed contact 11and the primary winding 8, thus causing a pulse to appear in winding 9,initiating the flash as described above.

The last switching position to be described, and called the secondswitching position herein, is the position wherein the contact 25 isconnected to the contact A, also designated by the numeral 40, by meansof the movable contact arm 21. Similarly, contact 36 is connected tocontact 42, also designated A, by means of the movable contact arm 23.

Considering first the input side, terminal 40 is directly connected to aterminal 41 which is the common point of the electrode 5 of flash tube 6and the resistor 39 which is connected in series with said flash tube.At the output side, the terminal 42 is connected to the primary winding43 of a quench tube ignition transformer 44 whose secondary winding 45has one terminal connected to the control electrode 46 of the quenchtube 47, while the other terminal is connected to the negative supplyterminal. The quench tube 47, one embodiment of terminating means, has afirst electrode 48 connected to the positive supply voltage and a secondelectrode 49 connected to the negative supply voltage. The quench tube47 is thus connected parallel to the flash tube 6.

In this switching position, the flash unit operates to terminateautomatically a flash originating in flash tube 6. The operation is asfollows.

The switch 11 is momentarily depressed, causing capacitor 7 to dischargevia primary winding 8, thus igniting flash tube 6 as described above.The voltage appearing across resistance 39 while the flash tube isignited, causes capacitor 27 to charge via diode 26 and thenow-connected switching contacts 40 and 25. At the beginning of theflash, the potential at point 41 rises rapidly and then decreases. Thuscapacitor 27 is charged relatively rapidly to its maximum value and thenremains at substantially with value since diode 26 prevents anydischarge of the capacitor when the voltage at point 41, that is thevoltage across resistance 39, decreases.

During the flash, the light reflected by the object to be photographed,falls upon photoresistance 28 which may, for example, be mounted on thefront surface of the electronic flash unit. Thus, the resistance ofphotoresistance 28 decreases allowing a larger current to flow from thecapacitor 27 to the integrating capacitor 29. This integration bycapacitor 29 of the individual current elements with respect to timecauses the voltage across the capacitor to rise slowly. This in turncauses the voltage applied to the control electrode 33 of thyristor 34to increase as described above in relation to the alternate modes ofoperation, and causes thyristor 34 to become conductive when apredetermined value (THRESHOLD VALUE) is reached. Capacitor 35, whichwas charged previously, then discharges via the thyristor and primarywinding 43 of the quench tube ignition transformer 44. The current pulsegenerated in the primary winding causes a corresponding current pulse tobe generated in the secondary winding 45 which is thus in the ignitioncircuit of quench tube 47, since it is applied between the electrode 49and the ignition electrode 46. Since quench tube 47 in the ignitedcondition has a considerably lower internal impedance than the flashtube 6 which is connected in parallel with it, the ignition of thequench tube causes the flash tube to be extinguished, since most of theenergy supplied by capacitor 1 is absorbed by the quench tube.

A second preferred embodiment of the present invention is shown in FIG.2. The circuit of this embodiment has been proven very effective inpractice. Since this circuit corresponds to a great extent to thecircuit shown in FIG. 1, corresponding parts in the two Figures havebeen given the same reference numbers. In the following discussion, onlythose circuit elements which are different for FIGS. 2 and 1, will bediscussed in combination with their operation in the overall circuit.

It will be seen that the switching means utilized in the presentembodiment, namely the embodiment shown in FIG. 2, differ somewhat fromthe corresponding switching means shown in FIG. 1. In particular, theswitching means comprise a slider 50 which simultaneously bridges anytwo sequential contacts of four fixed contacts labeled 53, 54, 55 and56. Similarly, at the output of the measuring and control circuit, theswitching means comprise a slider 51 mechanically coupled to slider 50which simultaneously bridges any two adjacent ones of four fixedcontacts 57, 58, 59 and 60. The switching means again have threeswitching positions, the first switching position corresponding to anignition of the flash tube 6 by externally supplied light, namely theposition wherein contacts 53 and 54; and 57 and 58 are bridged,respectively, by sliders 50 and 51; the second switching positionwherein automatic tennination of the flash generated by flash tube 6 isachieved, namely the position wherein contacts 55 and 56; and 59 and 60are respectively bridged by sliders 50 and 51. Finally, in the thirdswitching position, sliders 50 and 51 respectively assume the positionswherein contacts 54 and 55; and 58 and 59 are bridged.

As stated above, in the first switching position, which is shown in FIG.2, contact 51 bridges contacts 57 and 58, while contact or slider 50bridges contacts 53 and 54. This causes a connection to be achievedbetween a terminal 61 which is situated at the common point ofphotoresistance 28 and capacitor 29 to one terminal of a capacitor 63whose other terminal is connected to the base terminal 64 of atransistor 65. The base of transistor 65 is further connected to thewiper arm 31 of potentiometer 30. The time constant of the circuitformed by capacitor 63 and the part of the potentiometer 30 which is notbypassed by said capacitor, is chosen in such a manner that themeasuring and control circuit as shown in FIG. 1, responds relativelyrapidly to initiate a flash in response to externally furnished light,as for example light furnished by the main electronic flash unit.Transistor 65 serves to effect an impedance match between the measuringpart of the measuring and control circuit, which comprises thephotoresistance 28, and the control portion which comprises the controlcircuit of thyristor 34. The collector terminal 68 of transistor 65 isconnected to the common point of resistors 66 and 67 which form avoltage divider replacing resistance 17 of FIG. 1, via line 69. Theemitter terminal 70 of transistor 65 is connected to the controlelectrode 33 of thyristor 34. The voltage for driving the transistor 65is derived from potentiometer 30.

As mentioned above,'in the presently discussed switching position S,contacts 57 and 58 are bridged by slider 51. Since, a fixed connectionexists between contacts 58 and 59, the terminal of capacitor 35 which isnot connected to the anode of thyristor 34 is connected to one terminalof the first primary winding 71 of ignition transformer 72. The otherterminal of primary winding 71 is connected to the negative supplyvoltage, namely terminal 3. When capacitor 1 is charged, capacitor 35may in turn charge to a voltage corresponding to the voltage +U, minusthe drop across the primary winding 71, while the thyristor 34 isblocked. If now part of the light generated by the flash tube 6 fallsupon photoresistance 28, then, as previously explained in relation toFIG. 1, capacitor 29 continues to charge and the voltage appearing atterminal 61 is applied to the base of transistor 65 via line 62, slider50 and capacitor 63. The collector-emitter impedance of transistor 65then decreases and thyristor 34 becomes conductive. This allowscapacitor 35 to discharge via the thyristor, the bridged contacts 59, 58and 57 and the first primary winding 71 of transformer 72. The currentthus applied to primary winding 71 induces a voltage in the secondarywinding 73, which causes flash tube 6 to be ignited via its ignitionelectrode 13.

In the third switching position (position 0), contacts 54 and 55 arebridged by slider 50, while contacts 58 and 49 are bridged by slider 51.The electronic flash unit may, in this position, be used as an ordinaryelectronic flash wherein the flash is activated by means of theactivation of a contact, namely contact 11. This contact may beactivated in synchronism with the camera contact. It is true that inthis switching position, an operating voltage, derived from the mainsupply source, is supplied to the measuring and control circuit. Thus,light from a photoflash falling upon the photoresistor, might causethyristor 34 to become conductive. However, there is no connectionbetween contact 58 and the primary winding 71, thus preventing thecapacitor 35 from discharging. Quench tube 47 cannot be ignited either,since no connection exists between contacts 59 and 60, that is, there isno connection either between ignition capacitor 35 and ignitiontransformer 44. Thus, a flash can only be initiated by closing ofcontact 11, thus, analogous to FIG. 1, causing the ignition capacitor 7to discharge over a second primary winding 74 of ignition transformer72, whereby, in turn, a voltage is induced in the secondary winding 73,causing an ignition of flash tube 6 via the ignition electrode 13.

It will be noted that a DC voltage appears at circuit point 41 when theflash tube 6 is ignited. This voltage is applied to circuit point 61 ofthe measuring and control circuit via a decoupling resistor 75, thecontacts 54 and 55 which are bridged by slider 50, and line 62. However,this potential cannot contribute to the ignition of the quench tube,because the connection between the ignition capacitor 35 and theignition transformer 44 is broken by the fact that contacts 59 and 60are not bridged. The flash ignited by the closing of contact 11 thuscontinues without any termination by the quench tube 47.

In the second switching position (position A) automatic control of theduration of the light flash is achieved. Contacts 55 and 56 are bridgedby slider 50, while contact 59 and 60 are bridged by slider 51.

When contact 11 is closed momentarily, ignition capacitor 7 dischargesover the second primary winding 74 of ignition transformer 72. Thevoltage induced in the secondary winding 73 causes an ignition of theflash tube 6 via its ignition electrode 13. The voltage developed acrossresistor 39 when the flash tube is conducting allows a DC voltage toappear at circuit point 41 which is applied to the anode of diode 26 viathe decoupling resistor 75 and the bridged contacts 55 and 56. It willbe noted that diode 77 is connected in such a manner as to block currentflow over line 76 resulting from the potential at point 41. Thecapacitor 27, which has been charged to a DC voltage derived from thevoltage divider 18 and furnished via diodes 77 and 26, receivesadditional charge due to the DC potential mentioned above, when theflash tube is ignited. The resistance of photoresistor 28 is influencedby the light reflected from the object to be photographed, and thisresistor permits a proportionally larger current flow as the reflectedlight increases. Thus integrating capacitor 29 is charged. A determinedportion of the voltage developed across capacitor 29 is derived frompotentiometer 30 via the wiper arm 31 and is used as a control voltagefor transistor 65. If capacitor 29 has been charged to a particularvoltage, then thyristor 34 which is controlled by transistor 65, becomesconductive, thus furnishing a discharge circuit for capacitor 35 whichcomprises the cathode-anode circuit of the thyristor and the primarywinding 43 of ignition transformer 44. The current induced by thiscapacitor discharge in primary winding 43 is transferred to secondarywinding 45 and causes ignition of the quench tube 47 via the ignitionelectrode 46 of said quench tube. This causes the flash tube 6, which isconnected in parallel to the quench tube to be extinguished, thusterminating the light flash.

In some applications, it may be advantageous in both preferredembodiments (FIGS. 1 and 2) to furnish a different operating voltage forthe measuring and control circuit in dependence on whether the switchingposition S or A is being used. For example, in one of the switchingpositions, the operating voltage may be derived from resistor 18 asshown in FIGS. 1 and 2, while in the other switching position, thevoltage may be derived from a resistor which either is larger or smallerthan resistor 18.

While the invention has been illustrated and described as embodied in ameasuring and control circuit, it is not intended to be limited to thedetails shown, since various modifications, circuit variations andstructural changes may be made without departing in any way from thespirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention,and, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

1. Electronic flash unit for use in a photographic process, comprising,in combination, light-flash-generating means for generating a lightflash in response to a generating control signal applied thereto; meansconnected to said light-flashgenerating means for furnishing saidgenerating control signal under operator control; terminating meansconnected to said light-flash-generating means, for terminating saidlight flash in response to a terminating control signal applied to saidterminating means; measuring and control circuit means having aphotoelectric element mounted to receive light indicative of the lightutilized in said photographic process, said measuring and controlcircuit means furnishing received light signal when the quantity oflight falling on said photoelectric element has reached a predeterminedvalue; and switching means having a first state connecting saidmeasuring and control circuit means to said terminating means, and anexternally selectable second state disconnecting said measuring andcontrol circuit means from said terminating means and connecting saidmeasuring and control circuit to said light-flashgenerating means,whereby said received light signal functions as said terminating controlsignal termmatmg the flash generated by said light-flash-generatingmeans when in said first state and functions as said generating controlsignal initiating said light flash in response to externally furnishedillumination when said switching means are in said second state.

2. An electronic flash unit as set forth in claim 1, wherein saidreceived light signal is a pulse.

3. An electronic flash unit as set forth in claim 1, wherein saidswitching means comprise mechanical switching means, said first andsecond state respectively corresponding to a first and second switchingposition.

4. An electronic flash unit as set forth in claim 1, wherein saidphotographic process is the photographing of an object; and wherein thelight utilized in the photoelectric process is the light reflected fromsaid object.

5. An electronic flash unit as set forth in claim 1, wherein saidlight-flash-generating means comprise a flash tube, having a flash-tubeignition circuit, adapted to generate said light flash in response to acontrol signal applied to said flash-tube ignition circuit; and whereinsaid terminating means comprise a quench tube, connected in parallelwith said flash tube, said quench tube having a quench-tube ignitioncircuit, said quench tube being adapted to substantially short circuitsaid flash tube in response to a control signal applied to saidquench-tube ignition circuit.

6. An electronic flash unit as set forth in claim 5, wherein saidexternally furnished illumination is derived from a second electronicflash unit.

7. An electronic flash unit as set forth in claim 5, wherein saidswitching means are adapted to connect the output of said measuring andcontrol circuit means to said flash-tube ignition circuit when in saidfirst switching position, and to said quench-tube ignition circuit whenin said second switching position.

8. An electronic flash unit as set forth in claim 7, further comprisingfirst and second operating voltage furnishing means for furnishing anoperating voltage to said measuring and control circuit means, saidsecond operating voltage furnishing means being adapted to furnish saidoperating voltage only during the presence of a flash in said flashtube; and wherein said switching means are further adapted to connectsaid first operating voltage furnishing means to said measuring andcontrol circuit means when in said first switching position, and saidsecond operating voltage furnishing means to said measuring and controlcircuit means when in said second switching position.

9. An electronic flash unit as set forth in claim 8, wherein saidswitching means have a third switching position wherein said measuringand control circuit means is disconnected from both said first and saidsecond operating voltage-fumishing means.

10. An electronic flash unit as set forth in claim 7, wherein saidswitching means further have a third switching position wherein saidoutput of said measuring and control circuit means is disconnected bothfrom said flash-tube ignition circuit and said quench-tube ignitioncircuit.

1. Electronic flash unit for use in a photographic process, comprising,in combination, light-flash-generating means for generating a lightflash in response to a generating control signal applied thereto; meansconnected to said light-flashgenerating means for furnishing saidgenerating control signal under operator control; terminating meansconnected to said light-flash-generating means, for terminating saidlight flash in response to a terminating control signal applied to saidterminating means; measuring and control circuit means having aphotoelectric element mounted to receive light indicative of the lightutilized in said photographic process, said measuring and controlcircuit means furnishing received light signal when the quantity oflight falling on said photoelectric element has reached a predeterminedvalue; and switching means having a first state connecting saidmeasuring and control circuit means to said terminating means, and anexternally selectable second state disconnecting said measuring andcontrol circuit means from said terminating means and connecting saidmeasuring and control circuit to said light-flash-generating means,whereby said received light signal functions as said terminating controlsignal terminating the flash generated by said light-flashgeneratingmeans when in said first state and functions as said generating controlsignal initiating said light flash in response to externally furnishedillumination when said switching means are in said second state.
 2. Anelectronic flash unit as set forth in claim 1, wherein said receivedlight signal is a pulse.
 3. An electronic flash unit as set forth inclaim 1, wherein said switching means comprise mechanical switchingmeans, said first and second state respectively corresponding to a firstand second switching position.
 4. An electronic flash unit as set forthin claim 1, wherein said photographic process is the photographing of anobject; and wherein the light utilized in the photoelectric process isthe light reflected from said object.
 5. An electronic flash unit as setforth in claim 1, wherein said light-flash-generating means comprise aflash tube, having a flash-tube ignition circuit, adapted to generatesaid light flash in response to a control signal applied to saidflash-tube ignition circuit; and wherein said terminating means comprisea quench tube, connected in parallel with said flash tube, said quenchtube having a Quench-tube ignition circuit, said quench tube beingadapted to substantially short circuit said flash tube in response to acontrol signal applied to said quench-tube ignition circuit.
 6. Anelectronic flash unit as set forth in claim 5, wherein said externallyfurnished illumination is derived from a second electronic flash unit.7. An electronic flash unit as set forth in claim 5, wherein saidswitching means are adapted to connect the output of said measuring andcontrol circuit means to said flash-tube ignition circuit when in saidfirst switching position, and to said quench-tube ignition circuit whenin said second switching position.
 8. An electronic flash unit as setforth in claim 7, further comprising first and second operating voltagefurnishing means for furnishing an operating voltage to said measuringand control circuit means, said second operating voltage furnishingmeans being adapted to furnish said operating voltage only during thepresence of a flash in said flash tube; and wherein said switching meansare further adapted to connect said first operating voltage furnishingmeans to said measuring and control circuit means when in said firstswitching position, and said second operating voltage furnishing meansto said measuring and control circuit means when in said secondswitching position.
 9. An electronic flash unit as set forth in claim 8,wherein said switching means have a third switching position whereinsaid measuring and control circuit means is disconnected from both saidfirst and said second operating voltage-furnishing means.
 10. Anelectronic flash unit as set forth in claim 7, wherein said switchingmeans further have a third switching position wherein said output ofsaid measuring and control circuit means is disconnected both from saidflash-tube ignition circuit and said quench-tube ignition circuit.